The treatment of mixed municipal solid waste (“MSW”) presently most typically comprises passing that waste to some form of separation process by which organic materials therein are first separated, as much as possible, from inorganic materials. This initial separation step is invariably a size based separation, with organic material typically being smaller or softer than much of the inorganic material. The organic materials are subsequently directed, at least in part, to a rotting process, whilst the inorganic material is sorted into recyclables and non-recyclables, the latter being passed to landfill. The product of the rotting process is ideally a compost material and a biogas.
The efficiency of such processes are highly dependent upon the effectiveness of the manner in which the various separation steps are conducted. Further, the usefulness of the final products of such processes are dependent in large part upon their purity. For example, it is highly preferable if each of glass and grit, film plastics material and both ferrous and non-ferrous materials are removed from the organic material. However, there is invariably a compromise struck between the time taken to achieve a completely efficient result and the cost associated with such.
The composting processes utilised in traditional processes often produce odours which must be combated with expensive and complicated odour treatment arrangements if the treatment facility is anywhere near urban development. Alternatively, the treatment facility must be placed in very remote locations, which is not always possible or desirable.
Regarding the rotting process employed, it is known that solid organic waste material may be treated under either anaerobic or aerobic conditions to produce a bioactive, stable end product that, for example, may be used as compost for gardens or agriculture. This process is achieved through the action of, respectively, anaerobic or aerobic microorganisms that are able to metabolise the waste material to produce the bioactive, stable end product.
It is also known that the aerobic decomposition of solid organic waste material takes place in the presence of oxygen. The temperature of the waste material rises as some of the energy produced during aerobic decomposition is released as heat, often reaching temperatures of approximately 75° C. under ambient conditions. The solid end product is often rich in nitrates which are a readily bio-available source of nitrogen for plants, making the end product particularly suitable as a fertiliser.
It is further known that the anaerobic digestion of solid organic waste material takes place in the absence of oxygen. Anaerobic microbial metabolism is understood to be optimised when the organic material is heated to temperatures at which mesophilic or thermophilic bacteria are operative. The process of anaerobic microbial metabolism results in the production of biogas, in turn predominantly methane and carbon dioxide. The solid product of the process is often rich in ammonium salts. Such ammonium salts are not readily bio-available and are, consequently, generally treated under conditions in which aerobic decomposition will occur. In this manner the material is used to produce a product that is bio-available.
Typically, systems for the biodegradation of organic waste material are directed to either aerobic or anaerobic processes. However, there are a small number of systems that have sought to combine both anaerobic and aerobic biodegradation processes. The processes of German Patent 4440750 and International Patent Application PCT/DE1994/000440 (WO 1994/024071) each describe the combination of an anaerobic fermentation unit and an aerobic composting unit. Importantly, these systems describe discrete and separate vessels for the aerobic and anaerobic biodegradation processes.
International Patent Application PCT/AU00/00865 (WO 01/05729) describes an improved process and apparatus in which many of the inefficiencies of the previous processes and apparatus are overcome. The improved process and apparatus are characterised at a fundamental level by the sequential treatment of organic waste material in a single vessel, through an initial aerobic step to raise the temperature of the organic waste material, an anaerobic digestion step and a subsequent aerobic treatment step. During the anaerobic digestion step a process water or inoculum containing micro organisms is introduced to the vessel to create conditions suitable for efficient anaerobic digestion of the contents and the production of biogas. The introduced inoculum also aids in heat and mass transfer as well as providing buffer capacity to protect against acidification. Subsequently, air is introduced to the residues in the vessel to create conditions for aerobic degradation. It is further described that the water introduced during anaerobic digestion may be sourced from an interconnected vessel that has undergone anaerobic digestion.
In US Publication 20050199028 A1 there is described a method and apparatus for treating and recycling mixed municipal solid waste that is intended to minimise the quantity of waste passing to landfill. This involves biological treatment as a first step prior to subsequent separation steps to remove inorganic materials and recover recyclables. A further aerobic microbial treatment is provided before additional screening to remove inert compounds. A final washing step is used to remove salts from the composted organics. No provision is made for the removal of glass and grit in this method. Further, the first separator employed, in the form of a rotating drum, performs a limited size separation, thereby restricting the efficiency of the remainder of the method.
US Publication 20110008865 A1 discloses &method and apparatus for treatment of municipal solid waste in an effort to separate recyclables and to transform solid waste into energy and clean fuel. An initial autoclaving step is integral to the method and is aimed at breaking down fiber to fiber bonds of cellulosic material. A single trommel is used for separation and produces a homogenous organic fraction that is mixed with water from sludge dewatering. The organic stream undergoes fermentation and thermophilic anaerobic digestion. The methane produced is used to generate heat and electrical energy for plant operation. A thickened dewatered sludge is produced by the digesters that is intended as a feedstock for pyrolysis. The oversize from the trommel separation step is passed to steps in which metals, aluminium, glass and plastics are removed. The separation steps employed are coarse and relatively inefficient, including the fact that it is only the oversize from the trommel that is subjected to a number of the separation steps. No provision is made for the capture of organics that may have passed through the single trommel. Further, no provision is made for the separation of glass and grit. The method for solid waste separation of the present invention has as one object thereof to overcome substantially the abovementioned problems of the prior art, or to provide a useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.